The effect of increasing age on collateral ventilation between adjacent lung lobules will be investigated in three species, the horse, pig, and dog using excised lungs. Collateral ventilation will be investigated by measurements of quasi-static lung pressure volume curves, vital capacity, pulmonary resistance, and frequency dependence of compliance before and after acute small airway obstruction produced with plastic beads. Collateral flow resistance will also be measured. The objective of this research is to determine if the types of lung disease seen in these three species and at different ages in animals and man can be explained by the development of differing degrees of collateral ventilation. In a second part of the study, collateral ventilation will be measured in spontaneously occurring equine bronchiolitis and following chronic mechanical obstruction of the airways with beads. It is hoped this will determine if collateral ventilation increases subsequent to airway obstruction.